1. Field of the Invention
The present invention relates to a cathode ray tube, and more particularly, to a cathode ray tube capable of big-screen display, in which a plurality of divided regions of a phosphor screen are scanned separately, and images obtained in the individual regions are joined together to form a composite image, and a manufacturing method therefor.
2. Description of the Related Art
Recently, high-quality broadcasting and big-screen high-resolution cathode ray tubes therefor have been examined in various ways. Described in U.S. Pat. Nos. 5,365,142 and 5,287,034, Jpn. Pat. Appln. KOKAI Publication No. 5-36363, etc. are cathode ray tubes in which a plurality of divided regions of a phosphor screen with an integral structure are separately scanned with electron beams that are emitted from a plurality of electron guns.
The cathode ray tubes of this type are provided with an envelope including a faceplate and a rear plate, which are substantially flat in shape and are opposed to each other. A phosphor screen is formed on the inner surface of the faceplate. A plurality of funnels are fixed to the rear plate, and an electron gun is disposed in the neck of each funnel.
In these cathode ray tubes, a plurality of supports are arranged inside the envelope, in order to bear an atmospheric load that acts on the flat faceplate and rear plate. It is advisable to locate the supports so that they are in contact with a nonluminous section of the phosphor screen. According to the cathode ray tube described in U.S. Pat. No. 5,365,142, the phosphor screen is of a black-stripe type having stripe-shaped light absorbing layers (black stripe layers), and the supports are arranged so that their respective wedge-shaped distal end portions abut against the light absorbing layers.
In the case where the distal ends of the supports are thus arranged on the stripe-shaped light absorbing layers, they must be aligned accurately with the light absorbing layers. According to a positioning method described in Jpn. Pat. Appln. KOKAI Publication No. 7-78570, the supports are fixed to the rear plate in advance, and the rear plate, having the supports thereon, and the faceplate are provided with positioning means, individually. The respective distal end portions of the supports are aligned with the light absorbing layers of the phosphor screen by assembling the rear plate and the faceplate in predetermined relative positions, with the positioning means used as references.
In this assembly method, however, the respective positions of the supports relative to the phosphor screen are set indirectly by means of the positioning means that are attached individually to the rear plate and the faceplate. Accordingly, positioning the supports with respect to the phosphor screen with use of the positioning means involves the following problems.
(a) The supports are fixed with low accuracy. Since each support has its proximal end portion fixed to the rear plate, the positional accuracy of its distal end portion, which is in contact with the phosphor screen, is liable to be lowered due to fall or the like of the support.
(b) The accuracy of positioning by the positioning means is low. In general, the positioning means attached to the faceplate and the rear plate have a slidable fitting structure, and require a smooth sliding motion when they are positioned.
Further, the positioning means must be constructed so that they can undergo a smooth displacement attributable to thermal expansion without seizing, during a heat treatment for the formation of the envelope, which involves sintering of frit glass. Thus, a relatively wide clearance is required between the two positioning means. In this case, the feasible positioning accuracy is 0.1 mm or thereabout, which is inadequate for high-accuracy positioning. In consequence, a satisfactory accuracy cannot be enjoyed if this positioning structure is applied to a high-precision color cathode ray tube that has a phosphor screen whose phosphor layers and light absorbing layers are arranged at fine pitches.
(c) The positioning means cannot be easily applied to large-sized cathode ray tubes. With the increase of the screen size, the positioning means for keeping the faceplate and the rear plate in the predetermined relative positions are expected be tougher, so that their weight increases. In the case of a big screen, it is difficult, in view of both strength and accuracy, to position the two plates by using the positioning means that are only two or thereabout in number. It is necessary, therefore, to increase the number of the positioning means used for this purpose. Thus, the positioning means for maintaining the predetermined relative positions of the faceplate and the rear plate are not effective means to cope with the increase in size of the cathode ray tubes, entailing complicated construction, increased weight, and higher costs.